Exchange

Exchange is an alternate history novel where our risk-averse society has to cope with a frontier again, through a series of "Exchanges". An Exchange temporarily swaps town-sized pieces of an alternate reality where humans didn't make it through an ancient bottleneck with chunks our reality.  With little warning, a town can be in a land where sabertooths, giant bears and even more dangerous creatures still roam, a wild, dangerous place people can go to start a new life if they're brave or crazy enough.

When her town is caught in an Exchange, computer guru Sharon Mack has to fight giant predators, escaped convicts, and a mysterious cult to rescue her kidnapped daughter before the Exchange ends, trapping them forever.